High-temperature Tempering Research Articles

The novel process of spheroidizing-critical annealing used to optimize the properties of carburized steel and its effect on hardening mechanism of quenching and tempering

A new annealing process, spheroidation-critical annealing, was designed to optimize the properties of carburized steel 15CrNiMo instead of the traditional process. In this study, four factors and three levels of orthogonal experiment were used to design annealing experiments with different parameters, and the optimal combination of performance was obtained, giving consideration to both surface hardness and core toughness. After complete heat treatment, the vickers hardness of the surface of experimental steel could reach 752HV, and the impact energy of the core could reach 175 J/cm2. On this basis, the effects of new annealing process and traditional high temperature tempering process on the machinability of the experimental (the surface hardness of the experimental is taken as the performance index after the treatment of the two processes) are studied. At the same time, the effects of two processes on the surface hardening mechanism and core toughness of subsequent quenching and tempering processes of experimental steel are discussed. The results show that the new annealing process can hardly reduce the cutting property of the surface of experimental steel (after two kinds of process, the surface of the experimental steel vickers hardness is 302 HV and 325 HV), through the way of stable carbides, promote the quenching process of experimental steel surface high carbon martensite hardened, promoted 70 HV hardness compared with the traditional process. However, the toughness of the experimental core is insensitive to the two processes, so the new process can improve the surface hardness without reducing the toughness of the experimental.

Effect of quenching and tempering temperature on microstructure and tensile properties of microalloyed ultra-high strength suspension spring steel

Effects of quenching-tempering heat treatment on microstructural evolution and fracture behavior of microalloyed high strength suspension spring 55SiCrVNb were investigated. The results showed that an optimal combination of mechanical properties was obtained at the heat-treatment of oil quenching from 900 °C and tempering at 400 °C. Specifically, the ultimate tensile strength, yield strength, elongation and area reduction reached 2021 MPa, 1826 MPa, 10.3% and 42.7%, respectively. With the increasing austenitizing temperature, the grain size increased monotonically with more carbides dissolved in the matrix, and the strength first increased significantly and then decreased slowly. Furthermore, the fracture behavior transformed from ductile fracture to brittle fracture. In addition, the strengths were weakened dramatically as the tempering temperature increased but the elongation and area reduction were enhanced. Three kind of carbides are identified at different tempering temperatures, namely the coherent M2.5C and MC carbides gradually mature into large-sized non-coherent M3C and M7C3 carbides. Moreover, the dislocation reduction, lath boundary and twin martensite decomposition and carbide coarsening were exacerbated at higher tempering temperature while the fracture behavior changed from brittle fracture to ductile fracture.

Improvement of wear resistance in ferrite-pearlite railway wheel steel via ferrite strengthening and cementite spheroidization

It is important and necessary to improve the wear resistance of ER8 railway wheel steel without changing its hardness. In this paper, ferrite strengthening and cementite spheroidization were adopted to achieve the better wear performance in the improved wheel steel, i.e., HiSi steel. Firstly, alloying elements including Si, Mn and Ni were added to achieve ferrite strengthening. Besides, high-temperature tempering was applied to the HiSi steel to break the lamellar cementite into plates and spheroids. Subsequently, ER8 and HiSi steels were subjected to sliding wear test against to U71MnG steel. Finally, wear performance and relevant mechanisms were discussed in detail. Compared to the ER8 steel with lamellar cementite, the HiSi steel showed lower UTS of 846 MPa but higher elongation of 23.12% and Charpy absorbed energy of 37 J due to the spherical cementite. Moreover, less wear losses of the single pin and tribological pairs were also achieved in the HiSi steel. Adhesive wear and surface cracking were the dominant wear damage for both wheel pins. HiSi pin displayed more adhesive film, less and shallower surface cracks, thicker deformation layer and would suffer lower shear stress and normal stress. The better wear resistance of the HiSi steel can be attributed to the ferrite strengthening, less proeutectoid ferrite and granulate pearlite simultaneously.

Effect of heat treatment on the stress corrosion cracking (scc) susceptibiliy of the 13Cr martensitic stainless steel for steam turbine blade

Two different constant loads have been used to evaluate the effect of heat treatment on susceptibility to stress corrosion cracking (SCC) of 13 Cr martensitic stainless steel in 3.5% NaCl. The steel specimens were austenitized at 1050°C for 1 hour followed by oil quench to obtain martensite structure. Then, specimens were tempered at 400, 500, 600, 650 and 700°C for 1 hour. To investigate the SCC susceptibility, SCC tests were carried out at two different constant loads of 40% and 80% of ultimate tensile strength (UTS) in accordance with the method developed by Nishimura et.al. After the SCC tests, scanning electron microscope (SEM) observations of the fracture surface on the fractured specimens were conducted to investigate fracture mode. The experimental results showed that the 13Cr martensitic stainless steel was highly susceptible to SCC in a solution 3.5% NaCl at high constant load (80% of UTS) in almost tempered conditions. The least susceptible steel was the tempered one at 700°C. At lower constant load (40% of UTS), the highly susceptible steels were the tempered ones at 400 and 500°C. Steels which were tempered at high tempering temperature were not failed for 240 hours of SCC testing. The mechanism responsible for this susceptibility is thought to be secondary phenomenon where carbide precipitation of M7C or M23C6 occurs. The fracture surfaces exhibited intergranular cracks and dimple rupture fracture for steels tempered at 400, 600 and 650°C and transgranular cracks and cleavage fracture for steels tempered at 500°C.

Fracture Micromechanism of Cryogenically Processed Vanadis 6 Tool Steel

The effect of cryogenic processing and tempering on selected mechanical properties and fracture micromechanism of Vanadis 6 high alloy cold work tool steel was analysed. The samples were processed in cold nitrogen gas at -140 °C for 48 h, and tempered at temperatures of 170 – 530 °C. It was found that the hardness of sub-zero treated Vanadis 6 steel decreases with increasing tempering temperature. The highest hardness of the specimen 960 HV10 was achieved by tempering at 170 °C and the lowest hardness 790 HV10 resulted from tempering at the highest tempering temperature, i.e. 530 °C. However, the hardness of conventional heat treated samples was less than 800 HV10 in full range of tempering temperatures. The fracture toughness of sub-zero treated samples does not differ from what was obtained by conventional heat treatment schedule except the case of the high tempering temperature of 530 °C where an increase in fracture toughness by approx.. 3 MPa.m1/2 has been recorded. The carbides differ clearly in their role in the fracture propagation. While the secondary carbides undergo easily cleavage the eutectic carbides assist more probably decohesive fracture propagation.

Effect of Heat Treatment Conditions on Retained Austenite and Corrosion Resistance of the X190CrVMo20-4-1 Stainless Steel

In the present work the microstructural characterization of the powder-metallurgy X190CrVMo20-4-1 has been performed and correlated with its corrosion properties. The martensitic stainless steel was hardened at different austenitizing and tempering temperatures. Microstructural analyses were carried out using Scanning Electron Microscopy (SEM–EDS) to define the carbide distribution in the steel matrix. Carbides morphology and retained austenite content were evaluated and correlated to the corrosion behaviour of the different heat-treated steels, investigated by means of electrochemical tests. The results show the presence of M23C6 and M7C3 Cr-V based carbides homogenously dispersed in the matrix in annealed and quenching-and-tempering conditions. The carbides dissolution was evaluated by image analysis in every different heat treatment condition. When low tempering temperature was applied, an increasing in retained austenite content was defined by high austenitizing temperature and elevated carbides solubilization. At high tempering temperature, retained austenite content was not up to 5% nor affected by austenitizing temperature. Contrary to the expectations, HRC hardness was not influenced by the heat treatment conditions and retained austenite content. Corrosion resistance of the different heat-treated samples was found to be mainly influenced by retained austenite volume fraction and the tempering temperature. In particular, high austenitizing temperature and low tempering temperatures allowed the best corrosion resistance among the different heat treatment parameters investigated. The results obtained in the experimentation can provide support to the heat treatment optimization of the steel, widely used in tool and mould applications.

Hardness and Transverse Rupture Strength of TiC-Reinforced SKD11 Steel Matrix Composite

We investigate the effect of heat treatment on the hardness and transverse rupture strength (TRS) in the particulate TiC-reinforced SKD11 steel matrix composites, fabricated by a pressure infiltration casting process. A tempering process softens the composite as-quenched from the austenitization; however the hardness rebounds once the unstable austenite transforms into martensite upon cooling from higher tempering temperature. The fracture strength of steel matrix as well as the fraction and size of reinforcement is likely to affect the TRS of composite. For a given fraction of the TiC reinforcement, higher fracture strength of steel matrix is advantageous to achieve better TRS. Moreover, a refinement of TiC size makes the TRS of the composite more responsive to the change of fracture strength of steel matrix; increasing the fracture strength of steel matrix brings about more improvement of TRS. Thus, smaller TiC reinforcement size provides a better chance to control of TRS of the composite by altering the properties of steel matrix using a heat treatment.

Theoretical and experimental investigations of mechanical properties for polymorphous YTaO4ceramics

AbstractIn this work, the dense bulk polymorphous YTaO4ceramics with M or M' phase are synthesized by spark plasma sintering method accompanying with different tempering procedures. Combined with the nano‐indentation and theoretical calculation, their mechanical properties are systematically investigated. The identification of crystal structure reveals that the YTaO4crystallizes into M phase (space group:I2/a) with higher tempering temperature, otherwise it crystallizes into M' phase (space group:P2/a). The results of mechanical properties indicate M‐phase YTaO4possesses larger Young's modulus and hardness than that of M' phase. It is stemmed from the chemical bonding strength of M phase is stronger than that of M' phase, and the stronger bonding strength of M phase also results in its elastic resilience is superior to M' phase. Besides, on account of the low symmetry of monoclinic crystal system, the Young's modulus of polymorphous YTaO4ceramics exhibit strong anisotropy.

The effect of ɳ-Ni3Ti precipitates and reversed austenite on the passive film stability of nickel-rich Custom 465 steel

The multiple effects of ɳ-Ni3Ti precipitates and reversed austenite on the passive film stability of nickel-rich Custom 465 steel were studied. The results showed that with an increase in tempering temperature, the η-Ni3Ti precipitates gradually coarsened. The Volta potential of the ɳ-Ni3Ti/matrix interface was approximately 30 mV lower than that of the precipitates, which decreased the passive film stability. The volume of reversed austenite gradually increased at higher tempering temperatures. The Volta potential of the reversed austenite was 14 mV higher than that of the martensite, which enhanced the passive film stability and slightly increased the pitting corrosion resistance.

Mechanical Properties and Structure of a VNS9-Sh Steel as Functions of the Tempering Temperature

The influence of the tempering temperature in the range 125–700°C on the structure and mechanical properties of an austenitic–martensitic VNS9-Sh (23Kh15N5AM3-Sh) sheet TRIP steel is studied. A stable high level of the mechanical properties of the steel is found to be retained up to a tempering temperature of 450°C. At higher tempering temperatures, the mechanical properties decrease sharply against the background of the reverse transformation of strain-induced martensite into austenite. After tempering at 650°C and furnace cooling, the structure of the steel consists of α ferrite.

Application of −140 °C Sub-Zero Treatment For Cr-V Ledeburitic Steel Service Performance Improvement

The effect of −140 °C, 17-hour sub-zero treatment and subsequent tempering on the hardness, flexural strength, and fracture toughness of Cr-V ledeburitic steel was investigated. The main purpose was to highlight the role of microstructural alterations, such as a decrease of the retained austenite content, the quantitative parameters of carbide distributions, or others, due to applied treatments. Compared to conventional treatments, the applied sub-zero treatment improves the hardness of the material significantly within the whole range of tempering temperatures used. The hardness enhancement is in the range from 197 to 137 HV10 for tempering at 170 °C to 450 °C. The flexural strength is slightly improved at low tempering conditions up to 170 °C, while there is almost no effect of the sub-zero treatment on flexural strength for higher tempering temperatures. The fracture toughness is substantially improved (by about 3 MPa m1/2) in the untempered state as well as after high-temperature tempering. The fracture toughness is slightly worsened, by about 0.15 to 1 MPa m1/2, within the 170 °C to 450 °C range of tempering temperatures. These changes in fracture toughness are reached with a significant increase of hardness. This steel performance is associated with enhanced fracture surface roughness and an increased ductile microvoid coalescence micromechanism presence in fracture surfaces. The results also show that treatment at −140 °C leads to a greater enhancement of the complex of mechanical properties than can be obtained by soaking the samples in liquid nitrogen.

DEFORMATION AND FRACTURE OF STRENGTHENED HIGH-CARBON STEEL AFTER TREATMENT IN TEMPERATURE CONDITIONS OF PHASE PRE-TRANSFORMATION AND TRANSFORMATION

Traditional methods of heat treatment are energy-intensive and time-consuming, so the task of increasing their efficiency is very relevant. The process of repeated high-speed heating with short-term aging in the temperature range of polymorphic pre-transformation and transformation from the viewpoint of evolution of structure, properties and character of the fracture of quenched high-carbon steels was investigated. In particular, it was found that high-speed heating (600 – 700 °C/s) and short-term holding (0.5 sec) followed by cooling in salted water (6 °C) leads to the formation of a structure not differing from the structure of low-drawn (200 °C, 2 hours) martensite of traditionally hardened steel with 4 times increase in elongation and 2 times contraction while maintaining strength during the tensile test. Short-term aging of 8 – 15 – 25 seconds with repeated quenching of high-carbon steels from 820 °С in cold salted water (6 °С) leads to formation of the structure of ultra-fine-lamellar, submicroplast, globular perlite. There is a three-dimensional nanostructuring of steel that differs from traditional hardening with high temperature tempering by the structure and properties: magnitude of the applied stresses, both at deformation stages and at fracture (increase in σ by 55 %, in σ0.2 – by 17 %, in ψ – 8 times, in αn – by 80 %). Increase in short-time holding up to 40 – 50 sec, with repeated quenching from 820 °C leads, in contrast to the traditional one, to formation of the structure of ultra-small-malt martensite and to appearance in fracture on the slip planes of the patching structure, resembling a honeycomb in shape. After the low temperature tempering during the tensile test, all stages of deformation are presented. In fracture the crushing of pit structure and the absence of brittle tunnel pits are observed, plasticity is improved (δ ~ 1.5 times, ψ – 3 times) at strength preservation.

Kinetics of Residual Stresses in a Welded Frameless Tubular Node

The results of experimental studies of the variation of residual stresses in a frameless tubular node of the offshore stationary platform (OSP) due to technological effects on the metal are presented. It is shown in the paper that the value of the tensile residual stresses in sheet rolled products with 20-50 mm of thickness reaches 80-65 MPa. As a result of the subsequent technological redistribution, the residual stresses change in magnitude and sign, and reach 0.8-1.2 of the yield strength of the base metal. The most unfavorable from the point of view of the serviceability of OSP are the tensile residual stresses in welded joints, the maximum value of which is 350...500 MPa. High temperature tempering after welding does not always lead to the desired result: in the heat-affected zone of the corner joints of nozzles mounting to the waist tube of the node, the residual stresses amount to 120-150 MPa. Preheating temperatures in the range of 120-1600С do not have a significant effect on the value of residual welding stresses. For more effective reduction of σrs , its value should be 200-4500С, depending on the thickness of the joined parts and the strength class of the steel.

Scientific and technological bases for creation of cold-resistant steel with a guaranteed yield strength of 315–750 MPa for the Arctic. Part 1: Principles of alloying and requirements for sheet metal structure

The results of the choice of rational alloying and microalloying of cold-resistant steels with a guaranteed yield strength of 315–750 MPa are presented on the basis of the established interrelationship of phase transformations, structure, mechanical properties and performance characteristics when varying the content of basic alloying elements. Quantitative requirements for various structural parameters and their maximum permissible difference in sheet metal thickness up to 100 mm have been developed, depending on the strength category, manufacturing technology (thermomechanical treatment with accelerated cooling, hardening from separate furnace or rolling heating with high temperature tempering), which provide guaranteed characteristics of strength, cold resistance (impact work KV at test temperature –60 ... –80°С, critical temperatures of viscousbrittle transition Ткand zero ductility NDT) and crack resistance under the criterion of the critical opening in the top CTOD fracture.

Scientific and technological bases for creation of cold-resistant steel with a guaranteed yield strength of 315–750 MPa for the Arctic. Part 2. Technology of production, structure and properties of sheet hire performance

On the basis of the conducted research, a complex of scientific and technological methods has been developed for various technological processes (thermomechanical processing with accelerated cooling, quenching from rolling and separate furnace heating with high-temperature tempering). The developed method provides the formation of the structure of acceptable heterogeneity and anisotropy according to different morphological and crystallographic parameters throughout the thickness of rolled products up to 100 mm from low alloy steels with a yield strength of at least 315–460 MPa and up to 60 mm from economically alloyed steels with a yield strength of at least 500–750 MPa. The paper presents results of the industrial implementation of hot plastic deformation and heat treatment schemes for the production of cold rolled steel sheet with yield strength of at least 315–750 MPa for the Arctic. The structure of sheet metal thickness is given, providing guaranteed characteristics of strength, ductility, cold resistance, weldability and crack resistance.

Tempering of deformed and as-quenched martensite in structural steel

Annealing of deformed martensite (high-temperature tempering) in St37 steel was studied. Different reductions in thickness were considered and compared with the behavior of as-quenched martensite during tempering. tempering of the asquenched martensite was accompanied by the formation of carbide particles, incomplete disappearance of the lath martensite morphology, and continuous decrease in hardness until reaching low values. However, during tempering of the cold rolled martensite, the precipitation of carbides in the lamellar structure, development of distinct equiaxed ultrafine grains through a continuous recrystallization mechanism, and a sudden hardness drop were characterized. The importance of cold rolling reduction and its amount were also discussed.

Special aspects of thermal treatment of steel for hot forming dies production

Abstract It was found the influence of heat treatment parameters on the structure and properties of perspective die steel for hot working 70Cr3Mn2WTiB. It was specified the optimal mode of heat hardening for hot working hammer die from offered steel including spheroidizing annealing by the temperature of 780 °C with furnace cool up to 550 °C and then in a smooth air to the normal temperature; hardening by 1000 °C with cooling in oil; high-temperature tempering by 550–600 °C with cooling in a smooth air. During the hardening the formation of lath martensite with the high density of dislocation and the hardness HRC 57–62 occurs however it stays 0, 5–0,6% insoluble carbide phase. It’s shown that at the stage of high-temperature tempering by 550–600 °C in steel 70Cr3Mn2WTiB can be seen the process of hardness stabilization through the formation of special complex carbide intermetallic inclusions. The analysis of carbide phase by the temper showed that the combinations of type Me7C3 and Me3C are carbides the composition of which is changing during thermal influence. Thus, some atoms Fe and Cr are changed by titanium and vanadium atoms at different ratio. The simulation of temperature stressed condition of hammer die by thermal treatment was made. It’s shown that the offered mode of thermal treatment of steel 70Cr3Mn2WTiB leads to dispersed ferrite cementite structure with dispersed in it special carbides, high level of mechanical characteristics, provides homogeneous spreading of heat isotherms in extent of the hot deformation stamp and as a consequence insignificant inner residual strength. At the stage of high-temperature soak the temperature difference reduces to 25–30 °C.

Spheroidization heat treatment and intercritical annealing of low carbon steel

Spheroidization annealing of low carbon steel and its effects on the microstructure and mechanical properties of dual phase (DP) steel were studied. It was revealed that the reduction in strength and hardness of the quenched martensitic microstructure was much more pronounced compared to the fully annealed ferritic-pearlitic banded microstructure with spheroidizing time. This was related to the confinement of spheroidized carbide particles to distinct bands in the latter, and the uniform dispersion of carbides and high-temperature tempering of martensite in the former. During intercritical annealing of the spheroidized microstructures, the tendency to obtain martensite particles as discrete islands was observed. This, in turn, resulted in an inferior strength-ductility balance compared to the DP steel obtained from the intercritical annealing of martensite, which negated the usefulness of the spheroidized microstructures as the initial microstructures for the processing of DP steels.

Strengthening Mechanisms in Low Carbon Lath Martensite as Influenced by Austenite Conditioning

Low carbon lath martensitic microstructures are used in various steel products requiring high strength and toughness. These microstructures are conventionally produced through re-austenitizing and quenching followed by low or high temperature tempering. It is also possible to produce lath martensite through direct quenching immediately following thermomechanical processing. In this study, deformation below the austenite recrystallization temperature before quenching to form martensite was simulated through laboratory scale Gleeble processing of a 0.2 weight percent carbon ASTM A514 steel microalloyed with up to 0.21 weight percent niobium. Thermomechanical processing generally increases the dislocation density of the as-quenched martensite, which is sensitive to the austenite grain size before thermomechanical processing. The hardness of the thermomechanically-processed steels is generally greater than steels austenitized at comparable temperatures without deformation; this hardness difference is attributed to the increase in dislocation density and increased lath misorientation in the thermomechanically-processed conditions. The hardness is generally independent of prior austenite grain size for the thermomechanically processed conditions in contrast to conventionally austenitized and quenched conditions, which have a Hall-Petch correlation with austenite grain size. The strength increase of the thermomechanically processed conditions compared to the conventionally austenitized and quenched conditions is maintained after tempering. However, there is a larger drop in strength for small prior austenite grain sizes for both conventionally austenitized and quenched and thermomechanically processed steels. Overall, the strength of these lath martensitic steels can be directly related to dislocation density through a Taylor hardening model.

Effect of quench-tempering conditions prior to nitriding on microstructure and fretting wear mechanism of gas nitrided X210CrW12 steel

In this research the effect of quench-tempering conditions prior to industrial gas nitriding on the microstructure, fretting wear behavior and mechanism of nitrided X210CrW12 steel was investigated. The results reveal that higher tempering temperature can increase the thickness of the compound layer and the diffusion layer, and the amount of nitride precipitates also increases. The specimen tempered at the lower temperature has a thinner compound layer but a higher nitrided layer hardness. In addition, decarburization occurs during the nitriding process. The wear mechanism is also explained by the FEA wear model. The thickness of compound layer and the hardness of diffusion layer are the main factors that influence the wear behavior of the nitrided specimens. Increasing the wear resistance of counter material leads to the transition of wear mechanism from slight adhesion to severe abrasion. When the counter material is GCr15, all nitrided specimens present the similar wear rate values, while the wear rate of the specimen with the higher diffusion layer hardness and thinner compound layer tempered at the lower temperature of 550 °C is lowest when the counter material is SiC.